Air moving devices or blowers have been used for many years in conjunction with both large scale and small scale air handling installations, such as forced air heating, ventilating, and air conditioning systems. Often, in smaller residential installations, compact, lightweight, centrifugal fans are used for this purpose. Typically, those fans include a rotating member which includes a plurality of vanes or blades disposed radially from the central hub and curving backward from the direction of rotation in the familiar curved sunburst configuration. This rotating member is journaled in a generally circular housing which includes an exhaust port. Blower fans, such as this, are notorious for the extraneous sounds produced while the fan is in operation. Many people find these sounds to be unpleasant and annoying, so various efforts have been made to alleviate such noises. Such efforts have included various methods of dampening vibration and insulating the fan with a sound absorbing material. However, air moving blowers by their very nature cannot be completely insulated. One major source of noise associated with these fans is the impeller blades passing a discrete obstruction. Sound is emitted from the blower at a frequency proportional to the rotational speed and the number of impeller blades passing the obstruction. With respect to centrifugal fans, such an obstruction is created at the point where the exhaust port joins the main housing. This point, where air is forced to exit the housing, is commonly referred to in the art as the "cutoff". As each blade passes the cutoff it operates to compress air between the blade and the cutoff. Simultaneously, a region of relatively lower pressure is created behind the blade. As such, a cycle of compression/decompression results, thereby generating a pulsing noise.
Because this noise does not emanate from a mechanical or frictional contact, but instead involves a pulsing of air, conventional anti-friction and vibration dampening devices do nothing to reduce this noise. Further, because the source of the noise is in direct communication with the air handling system and is carried therethrough.
Additionally, the nature of the cutoff results in an inefficient transition for the air coming off the impeller blades to the exhaust port. This inefficiency results in lost pressure capabilities at low flow rates.
Accordingly, it has been desired to reduce the amount of noise produced at its source as well as to increase transitional efficiency, so as to increase pressure capability at low flow rates.